A modern felling head generally consists of a frame with a large accumulating area, a device for severing trees, and two sets of arms for gathering and holding trees. One arm/arms (the gather arm), is a rigid body that rotates about a point in the frame and is used for pulling trees into the accumulation area after they are severed. The other arm/arms (the bunching finger), is a hinged member that rotates about a different point on the frame and is used to hold severed trees in the accumulation area while another is being severed.
In operation, after the first tree has been severed and moved into the accumulation area by the gather arm the bunching finger is then actuated to come in contact with this tree and secure it in the accumulation area. The gather arm can now be opened so that another tree can be severed. As the second tree is severed, it is pulled into the accumulation area and pulled tight against the bunching finger which remains positioned in the accumulation area. The bunching finger is then retracted/reset. To accomplish this reset, the bunching finger has to fold in a manner to clear the first two trees that were severed and are being held in the accumulation area by the gather arm. Once the bunching finger has been opened a sufficient distance to clear the first and second trees severed, (still being held by the gather arm), the bunching finger reverses and engages both severed trees. This cycle continues until the accumulation area is full.
In the felling head as described, the bunching finger generally consists of inner and outer arms. The inner arm rotates about a point located on the frame. The outer arm rotates about the distal end of the inner arm. This rotation of the outer arm provides the folding motion to open and close entry to the accumulation area and thus hold the severed trees during the bunching process described above. The rotation of the inner arm of the bunching finger can undesirably change the position of the outer arm relative to the frame and such can change the amount of accumulation area the outer arm sweeps.
In order for a felling head, with the bunching finger and gather arm on opposite sides of the centerline of the machine, to properly sweep the accumulation area with the bunching finger, the inner arm must travel to the extent of its rotation towards the accumulation area before any relative rotation between the inner and outer arm occurs. Any deviation from this causes the outer arm to close prematurely resulting in an unsecured tree
Other designs have used different approaches to try to cause the bunching finger to have proper timing, such that there is generally no relative motion between the inner and outer fingers before the inner finger has reached the extent of its rotation. One such design utilizes a near over center condition to force the inner arm closed before the outer rotates. A precise arrangement of components is required to achieve this near over center condition. The result is a bunching finger that may or may not sweep the accumulation area correctly, given different static and dynamic variables.
A further drawback to the current design arises when the bunching finger is being reset after clearing the previously cut tree. In order to clear the previously cut tree as the bunching finger is being retracted, the inner arm must travel away from the extent of its rotation. At the same time, the angle of rotation between the inner and the outer arms is decreasing. This relative motion continues until the bunching finger has cleared the previously cut tree. The design then relies on an extension spring that is connected to the inner and outer arm to rotate the outer arm back to its near over center starting position.
During this period of time when the outer arm is being pulled back to its starting position, there is no force causing the inner arm to travel to the extent of it's rotation towards the accumulation area. This degree of freedom creates an infinite number of possibilities where the position of the inner arm relative to the frame is indeterminate. If the bunching finger's direction is reversed in one of these areas, the path of the bunching finger is unpredictable. The result is the possibility of the entire accumulation area not being swept and trees not being properly retained.
Essentially, the current configurations have no definitive way to return the inner arm to the extent of its rotation. While the extension spring causes rotation between the inner and outer fingers, which can help return the two to the near over center condition, due to geometry constraints, the only true over center position is when the cylinder is fully retracted. So in normal operation, the operator must fully open the arm, even though this may not be required to clear the tree, then close the arm tight against the trees in the accumulation area. This results in operational inefficiencies.
A further drawback of current designs is that the extension spring (to reset the position of the outer finger relative to the inner finger) suffers premature failures. Due to the geometry of the current design, this spring can be stretched to over 100% of its original length. The stress values generated and the frequencies they occur result in a failure that occurs often.